The prior art discloses a number of heat sealable plastic films having desired peel characteristics.
U.S. Pat. No. 6,590,034, issued to Wanic et al., discloses heat sealable and peelable films employing a blend of at least two immiscible polymers in the outer seal layer. Thus, the layer that provides the heat seal function is the one that is formed with the immiscible polymers to provide a weakened structure. This can adversely affect seal integrity.
Statutory Invention Registration H1727, to Jones et al., discloses a heat sealable, co-extruded multi-layer packaging film identified as having a peelable, easy-opening feature. In this structure, a first layer is provided with a blend of polypropylene and a primary resin, which can be a low density polyethylene. This first layer, which is described as being easily peelable, is disposed in contact with a second, outer, heat-sealable layer that is required to include polybutylene. As disclosed, the heat-sealant layer separates from the first layer at the interface thereof when a peeling force is applied to the film. The Jones et al. Statutory Invention Record does not disclose the relative amounts of polypropylene and primary resin included in the first layer. Also, the heat-sealing temperature of polybutylene, which is a required component in the heat sealant layer, shifts upward, i.e., increases, over time, thus making polybutylene an undesirable component in heat-sealant layers.
U.S. Pat. No. 5,997,968, issued to Dries et al., discloses a peelable, heat-sealable, multilayered polyolefin film in which two incompatible polyolefins are blended in an outer, peelable surface layer. The multilayer structure disclosed in the '968 patent includes at least three (3) co-extruded polyolefinic layers including a base layer, an intermediate layer and the blended, peelable surface layer, with the intermediate layer being between the base layer and the peelable surface layer. Thus, the peelable surface layer, which includes the blend of incompatible polymers, is the heat-sealable layer of the film and can adversely affect seal integrity.
U.S. Pat. No. 5,087,667, issued to Hwo, discloses a film having a peelable layer comprising a mixture of ethylene polymer, butene polymer and propylene polymer, and this peelable layer bonds to a polypropylene substrate by extrusion lamination without the need for an adhesive. There is no disclosure of blending incompatible polymers in any core layer to enhance, or control peelability.
U.S. Pat. No. 5,302,442, issued to O'Brien et al., discloses a film having an outer heat-seal layer comprising a ternary blend of a random ethylene-propylene copolymer, a butene polymer and a low density polyethylene. Separation of the film is described as taking place at the heat-seal interface between the heat-seal layers. This patent does not disclose a film having an incompatible blend of polymers in a core layer for the purpose of both weakening the core and providing a weakened interface between the core and heat-seal layer to provide controlled peelability at the interface and/or through the core, without adversely affecting seal integrity.
U.S. Pat. No. 5,912,084, issued to Hausmann et al., discloses a packaging seal layer, which is peelable, including a blend of: (a) a non-polar, non-ionomeric thermoplastic, which can be a polypropylene homopolymer, and (b) a polar ethylene copolymer having a carbon monoxide functionality and being present in an amount of 1-90% by weight based upon the weight of the blend. There is no disclosure of providing an incompatible blend in the core layer of a multilayer film for the purpose of providing controlled peelability without adversely affecting seal integrity.
U.S. Pat. Nos. 5,689,935; 5,829,227 and 6,203,867, all issued to Derkach et al., disclose a product package described as having reliable, controlled openability. The disclosed lamination is required to include an internal, three-layer, adhesive component providing internal adhesion between inner and outer film layers. The adhesive component includes outer layers of polyethylene and an inner polypropylene layer, wherein controlled opening is provided at one of the interfaces between the polyethylene layers and the polypropylene layer. The structures disclosed in the Derkach et al. patents are fairly complex, and do not provide controlled peelability at the interface between a heat-sealable outer layer and an adjacent layer to which it has an incompatible bond.
Although a variety of films have been disclosed for providing controlled peelability or openability in packaging constructions, a need is believed to exist for a simple film structure providing a controlled, reduced seal and peal strength but without adversely affecting the seal integrity provided between a heat-seal layer of the film and another surface. It is to such film structures that the present invention relates.